1. Field of the Invention
The present invention relates to a visible light communication system, and more particularly to a method of performing more accurate communication by providing a visibility link while data is transmitted/received.
2. Description of the Related Art
FIG. 1 is a flowchart illustrating a data transferring process via an Infrared Data Association (IrDA) link access protocol (IrLAP) in a general infrared wireless communication system. As shown, the data transferring process in a data line layer typically includes activating an infrared communication (step 100), discovering external infrared communication devices (step 102), negotiating a communication method between the discovered terminals (step 104), transferring data (step 108) through a link connection between transmitting/receiving terminals (step 106) after the communication method is determined, disconnecting the link connection for the communication (step 110) when the data transfer is completed, and deactivating the infrared communication (step 112).
As described above, the data link layer of an infrared communication system, i.e. an IrLAP link connection protocol, regulates infrared media access rules and infrared communication standards related to the procedure for a communication method between transmitting/receiving terminals. The protocol may be classified into three processes: a discovery process for discovering external infrared communication devices, a connection setup process, and a disconnection process.
FIG. 2 is a signal flowchart illustrating the discovery process in the IrLAP of an infrared communication system.
Referring to FIG. 2, an initiator 210 performs communication repeats to broadcast a discovery exchange station identification (D-XID) frame (steps 214, 216, and 220), and confirms whether a discovery response XID (D-R-XID) frame, which is a response signal to the D-XID frame, is received in a standby state for a slot period. Here, the slot indicates a period during which a half duplex type bidirectional communication occurs, where data flows in one direction or the other, but not both at the same time. If the slot is terminated, the initiator 210 writes information on the communication through a discovery log process. This operation is performed whenever the initiator 210 performs a specified transmission/reception; thus, it can be determined whether a service application intends to perform the communication.
A responder 212 recognizes the D-XID frame received from the initiator 212 (step 222), and transmits a D-R-XID frame, which is a response frame to the D-XID frame, to the initiator if the responder wants the communication.
The initiator 210 receives the D-R-XKD frame from the responder 212 (step 224), prepares a discovery log, reports the existence of the responder to a service application, and then shifted to a connection setup mode.
FIG. 3 is a signal flowchart illustrating the connection setup process in the IrLAP of an infrared communication system.
Referring to FIG. 3, the initiator 210 in the discovery process becomes a primary device 310, and the responder 212 in the discovery process becomes a secondary device 312. The primary device 310 transmits a set normal responder mode (SNRM) frame (step 311), and the secondary device 312, which has received the SNRM frame, transmits a response to the SNRM frame through an unnumbered acknowledgement (UA) frame transmission (step 316). The primary device 310, after receiving the UA, transmits a receive ready (RR) frame (step 318), completes a parameter setting required for the communication, and is shifted to a normal response mode (NRM) in which the communication can be performed.
FIGS. 4 and 5 are signal flowcharts in a normal response mode after the connection setup process in the IrLAP of an infrared communication system.
Referring to FIG. 4, a primary device 410 transmits an I-frame (step 420), and a secondary device 412 transmits an RR frame (step 424) as a response to the received I-frame. Here, as illustrated in FIG. 5, if a link between the primary device 510 and the secondary device 512 is disconnected, the I-frame is not transmitted to the secondary device 512, and thus the primary device 510 cannot receive the RR frame. In this case, the primary device 510 repeatedly retransmits the I-frame for a predetermined time even though it fails to receive the RR frame that is the response to the I-frame. If the predetermined time elapses, the primary device is shifted to a normal disconnect mode (NDM) to terminate the link setting.
As illustrated in FIGS. 2 to 5, the IrLAP of the infrared communication system is a half duplex type data link layer, i.e. an asymmetric data link layer. Hence, in the data transmission mode, the visibility is restricted depending on the amount of data of the transmitter and receiver sides. Also, since the IrLAP is a protocol using infrared rays, the application of the IrLAP to a visible light communication (VLC) protocol for the visibility is limited.
Further, according to the conventional infrared communication, infrared rays are not transmitted until the communication link is established. Thus, there is no way for the user to know if the link is disconnected halfway in the discovery process of the infrared communication devices, in the link connection process between two devices after the discovering, or in the data transferring process between two devices.
Accordingly, there is a need for an improvement in a local wireless communication system using a visible light to solve above drawbacks.